1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM), and more particularly to a structure of a magnetic memory cell composed by using elements capable of storing information of “1”/“0” by the so-called tunneling magneto-resistive effect.
2. Description of the Related Art
Recently, various memories for storing information by a new principle have been proposed. One of them is MRAM in which magnetic memory cells composed of magnetic tunnel junction (MTJ) elements for storing information of “1”/“0” by making use of the tunneling magneto-resistive effect (hereinafter referred to as TMR effect) are arrayed in a matrix. Examples of MRAM are disposed in U.S. Pat. No. 5,734,605 by Zhu et al. and also in U.S. Pat. No. 5,768,183 by Zhu et al.
In the MRAM, plural bit lines for writing/reading and plural write word lines are disposed in the orthogonal direction, and MTJ elements are disposed corresponding to each intersection. Each MTJ element is provided with a spin direction such that the long-side of rectangle is set along the write word line and that the short-side is set along the bit line, and along the long-side direction.
Each bit line is connected to each fixed layer of plural MTJ elements of the same row or same column, and each write word line is disposed so as to face closely to each free layer of plural MTJ elements of the same column or same row.
Writing to the MTJ element is achieved by passing current in the write word lines and bit lines and turning the spin direction of the MTJ element in parallel or anti-parallel direction by using the magnetic field generated by the current flowing in both wirings.
The distance between the write word line and the MTJ element must be longer than the distance between the bit line and the MTJ element, and more current must flow in the write word lines. The magnetic field generated from the write word lines distributes along a loop-shaped curve. Hitherto, since the magnetic field intensity applied in the central portion and in the end portion of the free layer of the MTJ element is different, the writing current increases due to an inverting magnetic field, which has been a problem for practical use.